Power generation systems such as gas turbines can operate on a wide variety of gaseous and liquid fuels. Modern high-efficiency combustion turbines combust fuels at temperatures that can exceed 2000° F. (1093° C.) and nominal firing temperatures continue to increase as demand for more efficient engines continues. The process of choosing a fuel is a complex task that is influenced by multiple factors including fuel price and availability, as well as government policy and regulation.
Opportunity crude oils, such as metal rich crude oils, provide a low cost liquid fuel option for power generation. However, operating a gas turbine with opportunity crude oils, or heavy fuel oil (HFO) obtained therefrom, can pose operational challenges. Opportunity crude oils may contain a variety of metal impurities that need to be treated prior to their combustion in order to attenuate the high temperature corrosion, erosion, and fouling effects of these metals on a gas turbine, which in turn may lead to turbine degradation and operability/reliability issues. One such impurity is vanadium.
In the gas turbine, combustion of vanadium containing fuels can create ash deposits comprising vanadium pentoxide (V2O5), which has a low melting point of about 1247° F. (675° C.). In the presence of low levels of sodium, V2O5 can form a series of low melting sodium vanadates such as Na2O—V2O5 which has melting temperatures of 1165° F. (629° C.). At typical gas turbine operating temperatures, these vanadic ash deposits are molten (e.g. liquid) and contribute to accelerated hot gas path hardware corrosion.
Vanadium is typically present as part of the heavy, oil soluble fuel component of crude oil. Vanadium derivatives in crude oil are organic in nature, and therefore cannot be removed with a water wash since they are not water soluble. Chemical additives have been added to fuel to inhibit vanadium corrosion, the most prevalent additives being magnesium containing additives. Magnesium modifies the ash composition by reacting with vanadium to form an inert magnesium vanadate compound, magnesium orthovanadate, which has an increased ash melting point of about 1965° F. (1074° C.). Vanadic corrosion is controlled because the combustion ash does not melt and remains in a solid state.
While magnesium has proved useful as a vanadic corrosion inhibitor, it suffers from many limitations. Combustion turbines cannot operate at firing temperatures above 2000° F. (1093° C.) when using magnesium additives to inhibit vanadic corrosion because magnesium orthovanadate has a limiting melting point. Additionally, the combustion ash deposits on gas turbine blades and vanes, leading to equipment degradation over time. Furthermore, in order to ensure complete reaction with vanadium, magnesium is fed in excess. This excess magnesium can sinter at high temperature and forms periclase within the deposit which is hard and not removable via periodic water washing of the hot gas path.